A MICROCRACK MODEL OF ROCK INELASTICITY PART I: FRICTIONAL SLIDING ON MICROCRACKS

A microscopically-based model of brittle-elastic behavior of compressed rock is constructed. The basic hypothesis is that frictional sliding on microcracks. accompanied by the growth of secondary tensile cracks, is the dominant mode of inelasticity at moderate compressive stresses (up to several hundreds of MPa (1)): microcracks (initially penny-shaped) are associated with weak grain boundaries and constitute a random field. This hypothesis is examined from the point of view of its ability to explain the basic features of macroscopic stress-strain behavior. In the present paper (Part I). we consider the first stage of inelasticity when driving force tau-mu sigma acting on cracks (tau and sigma are shear and normal tractions, mu is the friction coefficient) is positive but not high enough to initiate crack propagation. (The second stage - propagation of branched microcracks - is considered in Part II. see Kachanov. this issue.) Macroscopic stress-inelastic strain relations are obtained by summation of individual slidings. These relations are, generally. path-dependent, but there exists a cone of path-independence (in the stress space) which is quite wide, at least in the case of axisymmetric loading. Another feature of the mentioned relations is a significant stress-induced anisotropy. Although the overall inelastic effects are small in this stage. some of them are distinguishable and can be compared. at least qualitatively. with experimental data. The stress interval of frictional sliding (prior to onset of crack propagation) can be considerable.